auxin

Plant hormone that regulates stem and root growth in plants. Auxins influence many aspects of plant growth and development, including cell enlargement, inhibition of development of axillary buds, tropisms, and the initiation of roots. Auxin affects cell division mainly at the tip, because it is here that cell division in a stem or root mainly occurs. Just behind the tip the cells grow in size under the influence of auxins, causing the stem or root to grow longer. Auxin therefore affects the amount of elongation here too.

Synthetic auxins are used in hormone rooting powders to encourage cuttings to root. They are also used in some weedkillers, where high auxin concentrations cause such rapid growth that the plants die. Other uses include the prevention of premature fruit drop in orchards.

The most common naturally occurring auxin is known as indoleacetic acid, or IAA. It is produced in the shoot apex and transported to other parts of the plant.